Exploring the Impact of RIS on Cooperative NOMA URLLC Systems: A Theoretical Perspective

TL;DR

This paper provides a theoretical analysis of how reconfigurable intelligent surfaces (RIS) can enhance cooperative NOMA systems for ultra-reliable low-latency communications (URLLC), demonstrating significant BLER improvements through analytical expressions and simulations.

Contribution

It introduces a theoretical framework for integrating RIS with cooperative NOMA in URLLC, deriving closed-form BLER expressions, and exploring RIS zone division for performance gains.

Findings

RIS significantly reduces BLER compared to systems without RIS.

Maximum ratio combining (MRC) offers higher diversity gain than selective combining (SC).

Dividing RIS into zones further improves BLER, especially for cell-edge users.

Abstract

In this paper, we conduct a theoretical analysis of how to integrate reconfigurable intelligent surfaces (RIS) with cooperative non-orthogonal multiple access (NOMA), considering URLLC. We consider a downlink two-user cooperative NOMA system employing short-packet communications, where the two users are denoted by the central user (CU) and the cell-edge user (CEU), respectively, and an RIS is deployed to enhance signal quality. Specifically, compared to CEU, CU lies nearer from BS and enjoys the higher channel gains. Closed-form expressions for the CU's average block error rate (BLER) are derived. Furthermore, we evaluate the CEU's BLER performance utilizing selective combining (SC) and derive a tight lower bound under maximum ratio combining (MRC). Simulation results are provided to our analyses and demonstrate that the RIS-assisted system significantly outperforms its counterpart without RIS in terms of BLER. Notably, MRC achieves a squared multiple of the diversity gain of the SC, leading to more reliable performance, especially for the CEU. Furthermore, by dividing the RIS into two zones, each dedicated to a specific user, the average BLER can be further reduced, particularly for the CEU.